This patent specification relates to the field of optical fiber communications. More particularly, it relates to an optical fiber having a greater range of wavelengths and core diameters for which the optical fiber exhibits single-mode operation.
As the world""s need for communication capacity continues to increase, the use of optical signals to transfer large amounts of information has become increasingly favored over other schemes such as those using twisted copper wires, coaxial cables, or microwave links. Optical communication systems use optical signals to carry information at high speeds over an optical path such as an optical fiber. Optical fiber communication systems are generally immune to electromagnetic interference effects, unlike the other schemes listed above. Furthermore, the silica glass fibers used in fiber optic communication systems are lightweight, comparatively low cost, and are able to carry tens, hundreds, and even thousands of gigabits per second across substantial distances.
A conventional optical fiber is essentially an optical waveguide having an inner core and an outer cladding, the cladding having a lower index of refraction than the core. Because of the difference in refractive indices, the optical fiber is capable of confining light that is axially introduced into the core and transmitting that light over a substantial distance. Because they are able to guide light due to total internal reflection principles, conventional optical fibers are sometimes referred to as index-guiding fibers.
Conventional optical fibers have a solid cross-section and are made of fused silica, with the core region and the cladding region having different levels of dopants (introduced impurities) to result in the different indices of refraction. The cladding is usually doped to have a refractive index that ranges from 0.1% (single mode fibers) to 2% (multi-mode fibers) less than the refractive index of the core, which itself usually has a nominal refractive index of 1.47.
As known in the art, single-mode fiber is preferred over multi-mode fiber for high-capacity, long-distance optical communications. Single-mode fiber prevents electromagnetic waves from traveling down in the fiber in anything but a single, tightly held mode near its center axis. This is in contrast to multi-mode fiber, in which incident electromagnetic waves may travel down the fiber over several paths of differing distances. Accordingly, single-mode fiber allows for reduced group delay, and thereby allows optical signals to better keep their shape as they travel down the fiber.
FIG. 1 illustrates a cross-section of a conventional optical fiber 100 comprising a solid core region 102 surrounded by a solid cladding region 104. As described in Dutton, Understanding Optical Communications, Prentice-Hall (1998), which is incorporated by reference herein, at p. 45, optical fibers may be made single-mode by (i) making the core region thin enough, (ii) making the refractive index difference between the core and the cladding small enough, or (iii) using a longer wavelength. More particularly, as described in Hecht, Understanding Fiber Optics, Prentice-Hall (1999), which is incorporated by reference herein, at pp. 68-71, for a given propagation wavelength xcex, a maximum core diameter Dmax for single-mode operation is given by Eq. (1) below, where n1 is the refractive index of the core material, n2 is the refractive index of the cladding material can be represented by:                               D          max                =                              2.4            ⁢            λ                                π            ⁢                                                            n                  1                  2                                -                                  n                  2                  2                                                                                        {        1        }            
Also as described in Hecht, supra, for a given core diameter D, a cutoff wavelength xcexc below which propagation becomes multi-mode can be given by Eq. (2):                               λ          c                =                              π            ⁢                          xe2x80x83                        ⁢            D            ⁢                                                            n                  1                  2                                -                                  n                  2                  2                                                              2.4                                    {        2        }            
More generally, a condition for which single-mode propagation will occur can be stated in terms of the ratio of the core diameter D to the wavelength xcex according to Eq. (3):                               D          λ                ≥                  2.4                      π            ⁢                                                            n                  1                  2                                -                                  n                  2                  2                                                                                        {        3        }            
From a practical implementation perspective, it is desirable to make the diameter of the core region as large as possible while still maintaining single-mode operation in the wavelengths of operation. A larger core diameter allows for light to be more easily introduced into the fiber from light sources, thereby reducing the costs of both light sources and optical coupling equipment. A larger core diameter also allows for looser tolerances (i.e., reduced costs) in fiber splicing operations, and allows for other practical advantages. As indicated by Eq. (3) above, the maximum allowable core diameter increases as the refractive indices of the core material and cladding material get closer together. Of course, as these refractive indices get closer together, a corollary result is that the optical fiber may be made single-mode across a wider range of wavelengths for a fixed core diameter.
A problem, however, arises with conventional optical fibers in that current optical fiber manufacturing methods are restricted in their ability to precisely control the indices of refraction of the core material (n1) and the cladding material (n2). Because of this restricted ability, in commercially practical fiber the closeness of n1 and n2 is usually limited by design to no less than 0.1%. This, in turn, restricts the designed size of the core diameter for a given wavelength, and/or restricts the wavelengths of single-mode operation of a fiber for a given core diameter. For example, one common optical fiber manufacturing method referred to as flame hydrolysis uses a burner to fire a combination of metal halide particles and SiO2 (called a xe2x80x9csootxe2x80x9d) onto a rotating graphite or ceramic mandrel to make the optical fiber perform. See Keiser, Optical Fiber Communications, 2nd ed., McGraw-Hill (1991), which is incorporated by reference herein, at pp. 63-68. The index of refraction is controlled by controlling the constituents of the metal halide vapor stream during the deposition process. The process is xe2x80x9copen loopxe2x80x9d without a feedback mechanism to precisely control the ultimate index of refraction of the optical material. Moreover, the metal halide vapor stream is limited in its controllability and in its ability to control the ultimate index of refraction of the optical material.
Thus, the above flame hydrolysis technique and similar prior art methods used to vary the relative refractive indices of the core and cladding material, which are generally referred to as xe2x80x9cchemicalxe2x80x9d techniques herein, are generally limited in their ability to control these indices to closer than 0.1% from each other. Also, these techniques may introduce a substantial amount of unwanted impurities into the optical fiber, increasing Rayleigh scattering and reducing the quality and effectiveness of the optical fiber. Furthermore, a substantial degree of unwanted local or global variations in the doping may occur in the chemical deposition process and, because the optical fiber preform cannot be reheated to high temperatures without losing its desired refractive index profile, these variations remain in the final optical fiber and lessen its quality and effectiveness.
One wavelength band of great significance is the 1500-1610 nm band of operation of Erbium-Doped Fiber Amplifiers (EDFAs) used in most high-capacity, long-distance Dense Wavelength Division Multiplexing (DWDM) optical communications systems. Applying a refractive index difference of 0.1% between the core and the cladding and a wavelength of 1500 nm in Eq. (1), the maximum diameter of a conventional solid-core fiber would be about 17.8 xcexcm for single-mode operation; applying a wavelength of 1100 nm in Eq. (1), the maximum diameter of a conventional solid-core fiber would be about 13.1 xcexcm for single-mode operation. More commonly, a larger refractive index difference of 0.2% between the core and the cladding is used, for which the maximum diameter of a conventional solid-core fiber would be about 13.1 xcexcm for a wavelength of 1500 nm and 9.6 xcexcm for a wavelength of 1100 nm. For the conventional optical fiber 102 of FIG. 1, the core diameter is commonly about 9 xcexcm and the cladding diameter is commonly about 125 xcexcm. It would be desirable to increase the maximum core diameter of a single-mode fiber so that lower-cost optical sources, optical coupling components, and optical splicing components can be used therewith, and implementation can be otherwise improved.
Also using the best chemical techniques for a 0.1% refractive index difference between the core and cladding regions, Eq. (2) yields a cutoff wavelength of about 774 nm for the conventional optical fiber 102 of FIG. 1 having a core diameter of 9 xcexcm. For the more commonly used 0.2% refractive index difference between the core and the cladding, the cutoff wavelength would be about 1100 xcexcm. It would be desirable to further decrease the single-mode cutoff wavelength of an optical fiber.
Accordingly, it would be desirable to provide an optical fiber that, for a given wavelength of operation, provides single-mode propagation for larger core diameters.
It would be further desirable to provide an optical fiber that, for a given core diameter, provides single-mode propagation across an increased range of wavelengths.
It would be still further desirable to provide an optical fiber having increased precision in the relative refractive indices of the core and cladding regions to allow for closer designed differences therebetween.
It would be even further desirable to provide an optical fiber in which refractive index differences between the core and cladding regions may be achieved without the introduction of dopants, thereby allowing the optical fiber to comprise highly purified silica glass and to exhibit reduced dopant-induced adverse effects such as Rayleigh scattering.
In accordance with a preferred embodiment, an optical fiber having extended single-mode capabilities is provided, wherein subwavelength microstructural voids are introduced into the core and/or cladding to allow a fine tuning of the difference between their effective refractive indices. It has been found that the introduction of subwavelength microstructures into the optical material, preferably through a photolithographic process at the preform stage, allows for control of the effective refractive index difference between the core and the cladding that is more precise than the control afforded by chemical doping processes (e.g., flame hydrolysis) alone. The preferred embodiments take advantage of the fact that a preform slice exposed to a photolithographic process will have core and cladding regions that generally experience similar variations from nominal etching sizes during the process, and therefore highly precise area differences between voids in the core and cladding regions may be photolithographically achieved. Accordingly, the resulting effective refractive index difference between the core and the cladding may be made smaller, thereby allowing the optical fiber to exhibit single-mode properties for larger core diameters. In one preferred embodiment, for example, an optical fiber having a diameter of 25 microns and a single-mode cutoff wavelength of 1500 nm is provided. Advantageously, lower-cost optical sources, optical coupling components, and optical splicing components can be used with this optical fiber.
According to a preferred embodiment, a core portion is formed using a core material and a cladding portion is formed using a cladding material, the core and cladding materials having indices of refraction that differ by xcex94n percent or greater. Microstructural voids are formed in the core and/or cladding portions sufficient to cause their effective indices of refraction to be 0.5 xcex94n percent or less. In one exemplary embodiment, for example, a refractive index of the cladding material is 1.47, while the a refractive index of the core material is 0.1% greater at 1.47147. At the preform stage of optical fiber fabrication and prior to drawing, a void pattern occupying 10.0% of the cladding area is formed into the cladding, while a void pattern occupying 10.2% of the core area is formed into the core. The resulting optical fiber has a fine-tuned refractive index difference between the core and the cladding that is about 0.05%.
Any of several combinations of material refractive index selections and void area selections may be made in accordance with the preferred embodiments, provided that the resulting optical fiber has a core effective refractive index greater than the cladding effective refractive index. For example, the core material may be selected to have a refractive index less than the cladding material, with more and/or greater microstructural voids being introduced into the cladding than the core to reduce the cladding effective refractive index to less than (and within a close tolerance to) the core effective refractive index. In an alternative embodiment, the core and cladding materials may be identical, with a greater percentage of cladding cross-sectional area being occupied by voids than core cross-sectional area. Advantageously, according to this embodiment, no chemical doping is necessary at all, and the material may be heated prior to the photolithographic process to very high temperatures to remove impurities. The resulting fiber will comprise highly purified silica glass and will exhibit reduced dopant-induced adverse effects such as Rayleigh scattering.
According to a preferred embodiment, further precision in effective refractive index difference between the core and cladding may result from selective choices of the specific void patterns used. For example, to effectuate void ratios of 50.0% and 50.1% in the core and cladding, respectively, identical first patterns of larger circles designed to occupy 50.0% of the area may be formed in both the core and cladding. Then a second, distinct pattern of smaller circles may be formed in the cladding region designed to occupy 0.1% of the area. Because the core and cladding regions will generally experience the same variations in their first patterns of larger circles in the photolithographic process, and because any variations in the smaller circles have a smaller effect on the overall void area in the cladding, tolerance to variations in the photolithographic process may be further enhanced, thereby enhancing precision in the effective refractive index difference between the core and cladding regions.